Update level package in package.json (#588)

Update from 4.0.0 to 6.0.0 version. It fixes NR restarts (NR 502 errors) when trying to read from leveldb store after some time from writing.

Author: tmdoit

storage/leveldb/package.json

@@ -3,7 +3,7 @@
     "version"       : "0.1.0",
     "description"   : "A Node-RED node to read and write to a LevelDB database",
     "dependencies"  : {
-        "level"   : "^4.0.0"
+        "level"   : "^6.0.0"
     },
     "repository" : {
         "type":"git",